How to make these holes the same size?
You can start with a simple plane.

First set modifiers to it:


Mirror on X and Y
Array on X (merge option checked)
Array on Y (merge option checked)


In edit mode grab the plane to set the origin point to bottom left.

Poke face CtrlF then choose 'poke'.

Select the outer edges.

Subdivide them W then choose 'subdivide'.

Select the bottom left and upper right corner.

Scale them down using S.



The modifiers: